How to use Python readlines() to traverse two different files synchronously?

Question

I have two files:

test1.txt's content:

test_outside_1
test_outside_2
test_outside_3

test2.txt's content:

test_inside_1
test_inside_2
test_inside_3

Here is my Python code:

fobj1 = open('test1.txt', 'r')
fobj2 = open('test2.txt', 'r')

for aline in fobj1.readlines():
    print aline
    for bline in fobj2.readlines():
        print bline

the output I expected is:

test_outside_1
test_inside_1
test_inside_2
test_inside_3
test_outside_2
test_inside_1
test_inside_2
test_inside_3
test_outside_3
test_inside_1
test_inside_2
test_inside_3

But when I run my Python code the output is:

test_outside_1
test_inside_1
test_inside_2
test_inside_3
test_outside_2
test_outside_3

Could somebody tell me how to fix it?

Answer 1

When you do file.readlines() , you read the complete file and the cursor is at the end of the file, trying to call file.readlines() again on it would return an empty list, since there are no more lines to be read from the file.

One thing you can do is to store each in a list and iterate over the list. Example -

with open('test1.txt', 'r') as fobj1 , open('test2.txt', 'r') as fobj2:
    alist = fobj1.readlines()
    blist = fobj1.readlines()
    for aline in alist:
        print aline
        for bline in blist:
            print bline

Though please note reading complete files into memory may not be a good idea if the files are large. And there may be better ways depending on what your end goal is.

Answer 2

It's Occur because of in first iteration for test1.txt, test2.txt are read all then no other are exist for read test2.txt

You can also do it:

fobj1 = list(open('test1.txt', 'r'))
fobj2 = list(open('test2.txt', 'r'))
for aline in fobj1:
    print aline
    for bline in fobj2:
        print bline

Answer 3

As answer @Anand S Kumar in the first loop you read the completes files in one loop, the it return and empty list.

If you don't want to store all file (as said could be a bad idea) you will have to open again your file.

with open('test1.txt', 'r') as fobj1:
  for aline in fobj1.readlines():
    print aline
    with open('test2.txt', 'r') as fobj2:
      for bline in fobj2.readlines():
        print bline

but you will call a lot of time open.

EDIT: After commentary of #Anand is right.

If you want to keep memory low you should do something like

aline="."
with open('test1.txt', 'r') as fobj1:
  while aline != "":
    aline = fobj1.readline()
    print aline
    bline="."
    with open('test2.txt', 'r') as fobj2:
      while bline != "":
        bline = fobj2.readline()
        print bline